1. Field of the Invention
The invention relates to an intraocular lens for the human eye.
2. Description of the Prior Art
As intraocular lens implantation surgery has evolved, it has become clear that at this time, the most desirable location for an intraocular lens to be placed within the eye is in the posterior chamber. Furthermore, of the two locations available within the posterior chamber for placement of the intraocular lens, namely the ciliary sulcus or the capsular bag, it has been found most desirable to place the intraocular lens completely within the capsular bag. That is to say, the entire intraocular lens, including the optic and the haptic fixation members of the intraocular lens, is placed within the capsular "bag", the retained membranous capsular envelope which at one time contained the cataractous lens and is left behind as a result of an extracapsular or endocapsular cataract extraction. Ideally, the entire intraocular lens is placed and contained within the remaining capsular bag. This is desirable because it has been observed through clinical and pathological observation that the capsule isolates and fixates the intraocular lens from the surrounding reactive tissues of the eye making the implanted lens less irritating to the eye and better tolerated and centered. Also, it has been observed that asymmetric placement of the lens implant partially within the capsular bag and partially out of the bag in the ciliary sulcus results in a higher incidence of decentration of the lens optic causing optical and visual problems. Therefore, for reasons of lens tolerance and centration, complete symmetrical placement of the intraocular lens within the capsular bag is very desirable. Furthermore, as will be explained below, it has been observed that a large area of peripheral haptic-capsule contact even to the point of complete continuous 360.degree. circumferential contact is very desirable, if not the ideal. The historical developments in intraocular lens design leading to this conclusion are now presented.
The first modern intraocular lens design, which was developed and implanted by Ridley in November of 1949, was a posterior chamber intraocular lens intended for placement between the posterior capsule and the iris. Since that time, with some notable exceptions such as the designs of Pearce and Anis (U.S. Pat. No. 4,251,887), the more recent posterior chamber intraocular lens designs were also originally intended by their designers for placement within the posterior chamber in the ciliary sulcus, rather than within the capsular bag. The designs of Shearing, Sinskey, and Simcoe, which were developed in the latter half of the 1970's and the early 1980's when posterior chamber lens implantation was reintroduced and repopularized with modern surgical techniques, were initially intended for implantation in the ciliary sulcus. After clinical experience with posterior chamber implantation made it obvious that in-the-bag placement was more desirable than ciliary sulcus placement, these early posterior chamber intraocular lens designs intended originally for ciliary sulcus implantation were simply placed in the capsular bag without significant design modification. However, due to their limited design features, implantation, fixation, and centration within the capsular bag have been less than ideal.
It has become obvious from further clinical and pathological studies of lenses placed within the capsular bag that a large area of peripheral haptic-capsular contact is desirable for fixation and centration. This is in direct contrast to the earlier ciliary sulcus lens designs mentioned above which, due to the reactive nature of the ciliary sulcus tissue, strived for small rather than large areas of tissue contact to minimize ocular inflammation and thereby maximize ocular tolerance of the implant. It is now believed by some of the most respected investigators in the field of intraocular design, such as Apple, Galand and Anis, that the ideal in-the-bag intraocular lens design should incorporate the feature of continuous 360.degree. circumferential peripheral haptic-capsular contact for centration. This complete circumferential contact would then virtually eliminate any problems with decentration due to asymmetric contracture of the capsule postoperatively.
With modern intraocular lens production technology, several new design concepts incorporating continuous 360.degree. circumferential peripheral capsular contact have been developed and promoted, including the Galand disc lens and several designs by Anis. The disadvantage of the Galand disc lens is that its one-piece configuration is rigid and thereby requires a larger incision for implantation, both at the corneo-scleral limbus and in the capsular bag. Its rigidity threatens to tear or split the capsular incision during implantation resulting in serious clinical problems including vitreous loss and aborting of the posterior chamber implantation. These clinical problems have in fact been encountered resulting in attempts to modify the lens design by surrounding the central optic with a solid yet soft flexible material such as Silicone which might have less tendency to tear the delicate lens capsule during implantation. However, fixation and centration with this soft rim modification has been poor due to bending of the peripheral Silicone rim by the capsular contraction previously mentioned.
Anis has attempted to evolve the total circumferential contact design concept by developing designs which have 360.degree. capsular contact comprising a central optic joined to an outer circular circumferential ring or ring and partial plate. The advantage of this design is that it adds considerable flexibility, allows implantation through a smaller incision, and is less threatening to the capsule during implantation. However, the outer circumferential haptic ring, though flexible, is nevertheless fixed in that the overall linear circumferential dimension is fixed and unchangeable. That is to say, even if the circumferential ring haptic is compressed or distorted such that the "radial" dimension in one direction is narrowed or diminished, because the overall circumference is fixed and incapable of changing, another "radial" dimension of the once circular and now distorted haptic must now enlarge. Decreasing the radial dimension of a fixed circumference in one direction necessarily requires an increase of the "radial" dimension in another direction to allow the overall fixed circumferential length to remain the same. It is understood that all discussion of haptic loop flexion refers to flexion in the plane of the optic, and assumes the materials involved (usually polymethylemethacrylate or prolene (polypropylene) are essentially nonelastic and cannot stretch.
All of the above mentioned lens designs, and in fact all currently known intraocular lenses, have one or more of the following design inadequacies that makes them less than the above-stated ideal:
1. Inability to provide full 360.degree. circumferential capsular contact.
2. Inability to provide circumferential contact with an acceptable degree of flexibility.
3. Inability to adjust the overall linear dimension of the circumference (and radius) while still maintaining full 360.degree. circumferential peripheral capsular contact.
The inability to adjust the overall linear dimension of the circumference and radius while still maintaining full 360.degree. peripheral capsular contact is an especially significant and severe design limitation that puts any lens so designed at a noticeable disadvantage. This is because any such lens cannot conform to the different size capsular bags found in different eyes, or to a change in the capsular bag size resulting from capsular bag contraction within a given eye. Any lens that exhibits a fixed (although possibly flexible) overall effective circumferential and radial haptic dimension possesses this design flaw which limits its suitability for in-the-bag implantation.
To generalize further on this concept, it should be realized that it is not possible to create a 360.degree. continuous capsular contact haptic configuration which will allow for a change in the overall effective linear dimension of its circumference and radius if all of the junctions between the haptics are such that the "inner" haptic (that haptic portion which joins the optic but does not contact the peripheral capsule for fixation) and the "outer" haptic (that haptic portion in contact with the peripheral capsule for fixation) are joined by fixed, nonmoveable junctions. To put this more succinctly, no intraocular lens in which all of the haptic-haptic junctions are fixed and nonmoveable can simultaneously display both the characteristics of:
1. Continuous 360.degree. circumferential peripheral capsular contact, and
2. Variability in the linear dimension of that same circumference and corresponding radius. Stated another way, a lens must have at least one moveable haptic-haptic junction to display both of the above-mentioned characteristics.
As alluded to above, no known intraocular lens available possesses the features of both continuous 360.degree. circumferential capsular contact and a variable dimension circumference. In any lens which has 360.degree. continuous circumferential peripheral capsular contact by the haptic(s), if that circumference is to be variable, and in particular if it is to permit a reduction in the circumferential dimension, there must at some time occur a junction or even overlap of the haptics, and that junction or overlap must be nonfixed. The junction may occur between either "inner" and "outer" haptic portions as defined previously, or between two "outer" haptic portions. This moveable haptic-haptic junction has the property of allowing the haptics to overlap such that the effective circumferential dimension is adjustable and equal to the original circumference plus or minus the amount of overlap. Although one intraocular lens design has incorporated a moveable haptic-haptic design (U.S. Pat. No. 4,434,515), that lens design was a four point fixation style intended for anterior chamber rather than posterior chamber implantation, and in no way attempts or accomplishes 360.degree. circumferential capsular contact. It is the failure of all other known intraocular lens designs with encircling haptics to have this moveable haptic-haptic junction that is their considerable drawback.